The present invention relates to the simultaneous, in-situ determination of thermophysical properties, particularly of heat conductivity and thermal diffusivity, in the course of which a perturbated temperature field varying with time is produced in a certain volume of the material to be tested by heating, this temperature field is measured, then the required properties are determined by calculation from the obtained temperature data and heating power, as well as it relates to a measuring probe for the implementation of above process.
The importance of measuring heat conductivity and thermal diffusivity needs no verification. It is also known that in the case of inhomogeneous materials such as granulites, heaps of seeds, the earth's crust, or stratified rock mantle around an underground airway, the equivalent, averaged thermophysical properties can be measured most reliably in-situ, because they may be dependent on the concrete occurrences, pressures, moreover on the local moisture content. With sampling and laboratory measuring, systematic differences in the values to be determined may occur as compared to the in-situ state. All these emphasize the importance of the in-situ measurement in each case when a material of non-standard quality is in question. However, in the in-situ measurement technique only various attempts can still be observed. The same can be detected in the field of measurement technique applied to mine rocks: several ingenious ad-hoc methods are used for measuring the thermophysical properties of the rock mantle of airways in mines which can be summed up as follows:
One of the groups of the in-situ methods is represented by the short-probe transient heat conductivity measurements. This kind of measurement is performed by means of a probe equipped with linear heat source in its centerline. Thermocouples are arranged on the surface of the probe. The heat conductivity of the rock can be determined from the temperature rise measured within a time interval subsequent to switching on the heat source. Such solution is described in the UK Patent 2 071 319.
The drawback of the mentioned methods is that cylinder-symmetrical isotherms are assumed around the probe in the course of evaluation, but this symmetry is not necessarily true and it can be checked by measurement, since only the surface temperature of the probe can be measured instead of the full temperature field. The probe, however, is heated, hence the temperature difference between that of the probe surface and the rock should be reduced by careful probe installment but its complete elimination is not possible. These methods are applicable only to the determination of heat conductivity.
The second group of the in-situ measurement include the methods based on measuring and evaluation of transient cylinder symmetrical temperature distribution. This group of measurement is used for example in mines, when the temperature field around the mine airway is measured, as varying with the ventilation time, while the temperature distribution is measured in the radial bore holes characteristically 5 to 30 m deep (Hitchcock-Jones . . . Heat flow into a new main roadway Colliery Engineering, Feb.-Mar. 1985pp. 73-76 and 117-122, as well as Jones: Air temperature along a main intake roadway, Colliery Guardian, Jun. 1964, pp. 844-850). In this case it is assumed that the temperature of the ventilating air is constant during the whole time period of the measurement and a step-wise change in the air temperature was brought about at the very beginning of the measurement. This undoubtedly involves inaccuracies, since the measurement takes long enough time, generally several months. Ventilating air of varying temperature was assumed in other measurement methods and the evaluation was performed accordingly. Temperature changes with time recorded in at least two different depths of the rock wall are needed for the evaluation. (Cifka-Danko-Eszto: In-situ determination of the thermal diffusivity of rocks around underground airways. Publication of the Hungarian Central Institute for the Development of Mining, 1979, No. 22, pp. 133-138). On the other hand, others may use three or more different depths. (For example Vost, K. R.: "In-situ measurements of Thermal Diffusivity of Rock Around Underground Airways", Transaction of I.M.M., Vol. 85, pp. A57-A62.)
Measurements using fast heating of the rock surface are listed in the third group. Perturbations planned and performed carefully are applied to changing the temperature field. Consequently, the boundary conditions are known and a simple and accurate evaluation can be attained. For example, the fast heating of a closed roadway section was used and the temperature change with time of the rock was measured relatively close to the surface in the hole drilled into the rock surface. (Sherratt-Hinsley: A heating experiment to determine the thermal constants of rocks in-situ, The Mining Engineer, 1961, No. 3871, pp. 700-711). The method is suitable for the determination of both properties, i.e. heat conductivity and thermal diffusivity.
Experiments were conducted also by means of more indirect methods of in-situ measurement, for example the thermophysical properties are determined from the temperature rise of the air flowing along a roadway section of given length.
Such special measurements were also performed when in the bore hole drilled in the rock, the originally longitudinal temperature gradient is practically shunted with a probe of good thermal conductivity and the change of the temperature gradient is measured (U.S.Pat. No. 3 808 889).